1. Field of the Invention
The present invention relates to a radar system for use on cars and radar modules thereof, and in particular to a small-sized and low production cost radar module and radar system.
2. Description of the Related Art
A radar system loaded on cars, such as passenger car, is utilized in combination with an alarm device or display device for avoiding collision. In such a car-loaded radar system, detectability for a short distance on the order of tens of centimeters is required since a rear-end impact in a short distance between cars during traffic congestion or near collisions during putting a car into a garage is also a target to be warned, and hereby the shape of FM radar signal is more suitable than that of pulse radar. Also in miniaturizing a module, using a millimeter electric wave having a high frequency is preferable. Such a millimeter wave FM radar system is disclosed in the specification of U.S. Pat. Nos. 5,181,037 and 5,229,774 related to the prior application of the present applicant.
Generally, in an FM radar system, the frequency of a beat signal is arranged to rise in proportion to the distance to an object that generated a reflected wave. Accordingly, with shorter distance to the object, the frequency of a beat signal lowers and becomes undetectable under disturbance of 1/f noise which is generated in a mixer. In an FM radar system according to the prior patents mentioned above, to reduce the effect of 1/f noise by raising the frequency of a beat signal, a heterodyne method for modifying the frequency of a local signal is employed or a delay line is inserted. However, a heterodyne method requires a local oscillator used for frequency conversion and accordingly becomes expensive, whereas a method for inserting a delay circuit has a size problem in that an inserted delay line requires a module to be larger in size.
Furthermore, an FM radar system according to the prior patents mentioned above is arranged to switch a transmitter step multiplier, a circulator, antennas used in common for transmission and receiving, a mixer and a receiver step multiplier constituting each FM radar module by a synchronous operation between a transmitter side switch and a receiver side switch. Thus, as many transmitter-receiver sections as antennas become necessary and the number of parts increase, so that there arise problems that the whole FM module does not only become large in size but production cost also increases considerably.